1. Field of the Invention:
This invention relates to an optical memory device which conducts at least one of a recording operation, a regenerating operation, or an erasing operation of information by means of light. More particularly, it relates to the shape of guide tracks and/or track addresses of the optical memory device.
2. Description of the Prior Art:
Optical memory devices have come to public notice as a dense and mass memory device. They can be classified into three groups consisting of regenerative memory devices, additional recording permit memory devices and rewrite permit memory devices. Optical memory devices, which can be classified into additional recording permit memory devices and rewrite permit memory devices, generally have guide tracks to guide a light beam for recording and/or regenerating information to a given portion of the optical memory device. A portion of each of the guide tracks can be formed into a pit-shape, resulting in a track address. For example, the optical memory disk shown in FIG. 1 comprises a substrate 1 and a plurality of guide tracks and track addresses 2 and 3 , both of which are concentrically or spirally formed on the substrate 1. (A single guide track 2 and a single track address 3 are only illustrated herein for easy understanding.)
FIGS. 2 and 3 show the guide tracks 2 and the track addresses 3. The depth d of the guide tracks 2 is selected to be .lambda./8n to maximize the quantity of tracking servo signal, while the depth d' of the track addresses 3 is selected to be .lambda./4n to maximize the quantity of track address signal, wherein .lambda. is the wavelength of laser light, etc., used for the optical memory device and n is the refraction index of the portions of the guide tracks 2 and/or the track addresses 3 of the substrate 1.
FIG. 4 shows a recording medium layer m on the substrate 1. A laser beam 4 is radiated in a spot fashion to the recording medium layer m, resulting in a variation in the reflectivity of the recording medium layer m. The variation is retained thereon as a record of information based on the laser beam 4. When the laser beam 4 is later radiated in a spot fashion to the recording medium layer m in the same manner as above-mentioned, the variation can be read out and regenerated as a signal.
The conventional shape of the guide tracks 2, which is determined in such a manner to maximize the quantity of the tracking servo signal, is not necessarily advantageous for recording and/or regeneration of information. For instance, a regeneration signal from the recording medium layer m on a flat substrate having no guide tracks 2 has a more excellent quality than that from the recording medium layer m on a rugged substrate having guide tracks 2, because, although the guide tracks allow for an increase in the quantity of signal, the edges of the guide tracks cause noise in the signal. Since the guide tracks are essential to optical memory devices which are of additional recording permit memory and/or rewrite permit memory types, noise due to the shape of the guide tracks must be minimized and the quantity of signal therefrom must be maximized.
On the other hand, the shape of the track addresses 3 (shown in FIG. 2) is determined in light of both the ease or difficulty of the formation thereof and the quantity or quality of address signal, and thus the depth d' thereof is not necessarily required to be .lambda./4n.